1. Field of the Invention
The present invention relates generally to low-noise block downconverters (LNBs) and more particularly to LNBs for satellite communication systems.
2. Description of the Related Art
Various satellite communication systems provide communication signals (e.g., television signals) directly to subscriber locations (e.g., homes, apartment houses and offices). In an exemplary system, a satellite radiates microwave signal beams (e.g., in C-band or Ku-band) and the satellite""s transmit antennas are generally configured so that their transmit beams illuminate one or more predetermined coverage areas on the earth. In such a coverage area, the signals are received in a system subscriber""s antenna and typically downconverted to an intermediate-frequency signal band before further downconversion and detection at a lower detection frequency.
Different satellite transmit frequency bands have been assigned in different regions of the world. An exemplary transmit band 11 is shown in the frequency plan 10 of FIG. 1A. Upon receipt, signals in this transmit band are typically block downconverted to a first intermediate-frequency (IF) band 12 and then selected channels are further downconverted to a second IF region 13 for demodulation. The second downconversion and demodulation are generally performed in an interface module such as an integrated receiver decoder (IRD).
As shown in FIG. 1B, the transmit band (11 in FIG. 1A) is generally divided into transmit channels 14 of various channel widths (e.g., between 6 and 40 MHz) and these transmit channels are typically separated in frequency by guard bands 15. To increase their number, the transmit channels are typically grouped in two channel sets 16 and 18 wherein each channel set basically spans the same transmit band. As indicated in FIG. 1B, channel isolation is enhanced by centering each channel of one set on a boundary between adjacent channels of the other set.
The channel sets are further isolated by transmitting one channel set with a selected electromagnetic polarization and transmitting the other channel set with a different electromagnetic polarization. For example, the channel sets can be transmitted with vertical and horizontal polarizations or with left-hand and right-hand circular polarizations. Essentially, polarization isolation facilitates a dual use of the transmit band.
The generic description above is realized with different specific communication system names, frequencies and signal polarizations in various parts of the world. Specific system names include direct broadcast systems (DBS); direct service satellite (DSS), direct to home (DTH) and fixed service satellite (FSS). Specific frequencies of the transmit band (11FIG. 1A) include 11.7-12.2 GHz, 12.2-12.7 GHz and 12.25-12.75 GHz,. Specific frequencies of the first IF band (12 in FIG. 1A) include 950-1450 MHz and specific frequencies of the second IF region (13 in FIG. 1A) include 70 MHz. Specific signal polarizations include vertical and horizontal polarized signals and clockwise and counterclockwise polarized signals.
A polarization-sensitive antenna must be used to receive and detect the polarized channel sets of a satellite communication system. Once the transmitted signals are detected in an antenna, the polarization isolation is lost and must be replaced with other isolation measures in further processing and distribution of the IF signals.
In one conventional receiving system, the channel sets (16 and 18 in FIG. 1B) are detected on different probes of a polarization-sensitive antenna and a switch couples a selected one of the probes through a low-noise block downconverter (LNB). A subscriber""s selection of a channel causes an IRD, for example, to direct the switch to a channel set that contains the selected channel.
Isolation between channel sets is maintained in this system because only lone detected channel set is directed into the LNB at any given time, i.e., the downconverted channel sets are isolated because they are temporally separated. Although temporal isolation facilitates the use of a single-cable distribution network and is satisfactory for providing television channels to a single television set, it forms a limited system for subscribers who have multiple television sets or for multisubscriber installations (e.g., apartment houses) because, at any given time, it restricts all viewers to use of a single channel set.
In another conventional receiving system, two LNBs are provided with a common local oscillator signal and each channel set is detected and passed through a respective one of the LNBs so that both channel sets are simultaneously available at the two LNB output ports, i.e., the downconverted channel sets are isolated because they are spatially separated. Although both channel sets are simultaneously available in this system, an expensive dual-cable distribution network and a switching system are generally required to distribute them to subscribers.
The present invention is directed to methods and systems that facilitate the use of inexpensive single-cable distribution networks to simultaneously deliver all transmitted communication channels of a direct broadcast satellite system to a plurality of subscriber locations.
These goals are realized with methods that spectrally separate first and second satellite signals which occupy a common frequency band with different first and second electromagnetic polarizations. In particular, these methods include the steps of guiding the first and second satellite signals respectively along first and second signal paths, translating the first and second satellite signals to different first and second intermediate-frequency bands and coupling the first and second satellite signals from the first and second signal paths to a common third signal path.
These goals are further realized with a frequency-converter system that includes first and second low-noise block downconverters whose outputs are coupled into an output diplexer. The low-noise block downconverters each have a downconverter mixer and a local oscillator that couples a local oscillator signal to the mixer but the local oscillator signals are sufficiently spaced apart to convert the first and second satellite signals to spectrally-spaced first and second intermediate-frequency (IF) bands. Because the IF bands are spectrally separated, they are isolated and can be combined in the diplexer and distributed to subscribers over an economical single-cable distribution network. A frequency-converter system embodiment includes a polarity-sensitive antenna such as a reflector and a feed horn that has orthogonally-arranged first and second probes which are respectively coupled to the first and second low-noise block downconverters.
The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings.